This invention relates to an oxidation catalyst deposited on a substrate for the removal of CO, hydrocarbons and particulate matter from an exhaust stream, particularly a diesel engine exhaust stream, and a process for its use. The invention particularly relates to an oxidation catalyst deposited on a substrate, wherein the oxidation catalyst includes a platinum group metal supported on a refractory oxide, tin oxide, particularly large particle size tin oxide, and a zeolite. The invention more particularly relates to the above-referenced oxidation catalyst deposited on a substrate, wherein the refractory oxide is modified by blending it with a rare earth metal oxide.
Diesel engine exhaust emissions often include incomplete combustion products, such as hydrocarbons, particulate matter, CO, nitrogen oxides, and sulfur oxides. The particulate matter is generally a combination of condensed heavy hydrocarbons from unburned fuels and lubricants, carbon soot and sulfuric acid. As a result of the presence of these components, emissions from diesel engines can create serious air pollution which has a negative impact on human health. Accordingly, it is important that these pollutants from diesel engine exhaust emissions be trapped and/or converted into harmless compounds.
Such exhaust gas pollutants are commonly trapped by filtration with a heat-resistant exhaust gas filter, such as a ceramic foam, wire mesh, metal foam or a wall-flow type ceramic honeycomb filter. These filters capture the fine, carbon-based particles and can be regenerated by burning the accumulated carbon particles by use of a burner or the like when pressure drop within the system occurs. Alternatively, these filter can be produced from a heat resistant, gas filter structure whereby both the filtration operation and the regeneration treatment are performed at the same time and thus, no separate regeneration step is necessary.
These exhaust gas filters are generally coated with a catalytic material which tends to oxidize components of the exhaust stream. A common active component for these catalysts is a platinum group metal, such as platinum, palladium and/or rhodium. Such metals are preferred because of their high hydrocarbon oxidation activity at relatively low temperatures.
The platinum group metal component is often supported on a refractory inorganic oxide material, such as alumina, silica, titania, and zirconia.
Other catalytically active materials may be incorporated with the platinum group metals on the exhaust gas filter including compounds containing elements from the group of vanadium, iron, cobalt, nickel, molybdenum, tungsten, niobium, zinc, tin, copper, chromium, manganese, cerium, lanthanum, silver, barium, magnesium, calcium, strontium, potassium, sodium, cesium, and rubidium. In one example coated diesel particulate filters utilizing a combination of metals comprising a platinum group metal and a silver vanadate, are disclosed in U.S. Pat. Nos. 4,510,265 and 4,477,417.
U.S. Pat. No. 4,749,671 teaches an exhaust gas catalyst composition composed of a refractory, three-dimensional structure with a catalytically active substance secured thereto. The catalyst is designed to treat CO, hydrocarbons and fine carbonaceous particles in an exhaust gas from an automobile engine, particularly a diesel engine, at lower temperatures. The refractory, three-dimensional structure taught is selected from a ceramic foam, an open-flow ceramic honeycomb, a wall-flow honeycomb monolithic body, a metal honeycomb or a metal foam.
U.S. Pat. No. 5,628,975 teaches a method for purifying exhaust gases from a diesel engine by passing the exhaust gases through a honeycomb catalyst of specified geometry. This honeycomb catalyst comprises a flow-through metal or ceramic honeycomb carrier having 250 cells per square inch parallel to the direction of flow of the exhaust gases and at least one catalytically active component deposited thereon, selected from the group consisting of platinum, palladium, rhodium, iridium, vanadium, molybdenum, copper, silver, gold, rare earth elements and partially substituted perovskite composite oxides having an oxygen vacancy. Optionally, at least one refractory inorganic oxide, selected from the group consisting of alumina, silica, titania, zirconia, and zeolite, can also be added.
Although these systems have been useful for the treatment of diesel exhaust emissions in the past, there is still a need for an improved system for the removal and/or oxidation of hydrocarbons, particulate matter, such as soot, and CO.
It is therefore an object of the invention to disclose an improved oxidation catalyst deposited on a substrate utilized for the purification of exhaust gases, particularly diesel engine exhaust emissions.
These and other objects can be obtained by the catalyst deposited on a substrate, process for the preparation of the catalyst and catalyst substrate, and process for the use of the catalyst and catalyst substrate for the purification of exhaust gases, particularly diesel engine exhaust gas streams of the invention.